1. Field of the Invention
The present invention relates to microwave powered lamps and more specifically, to microwave powered lamps having reliable detection of bulb failure or degrading of performance.
2. Description of the Prior Art
FIG. 1 illustrates a prior art microwave powered lamp 10 of the type sold by the Assignee of the present invention and as described in the Assignee's U.S. Pat. No. 6,445,138 which is incorporated herein by reference in its entirety. The microwave-powered lamp may be used to produce ultraviolet (UV) or visible light depending on the application such as, but not limited to, curing surface coatings. A magnetron 12 provides microwaves transmitted through a microwave cavity/waveguide 14 to a microwave excited bulb 16 which outputs light, as stated above, in either the UV or visible spectrum depending upon the application. An air source 18 blows air 20 through a housing 22 which contains the magnetron 12, microwave cavity/waveguide 14, and microwave excited bulb 16. As indicated, air 20 flows through the housing around the magnetron 12 to provide cooling thereof and into the microwave cavity/waveguide 14 into and around the bulb 16 to provide cooling of the bulb. The lamp housing 22 is designed to channel air 20 in contact with cooling fins 23 of the magnetron 12 through openings 24 and then through openings (not illustrated) in the reflector 26 past the bulb 16 as described above and out of the housing 22. The air 20, which is heated by the magnetron 12 and the bulb 16, exits through the opening 29 which is covered by a microwave retaining screen (not illustrated) through which the light is outputted after being reflected by a light reflective cavity 28.
The magnetron 12 is subject to damage if the light bulb 16 becomes inoperative. An assembly of a photocell 32 and an associated circuit 33, which senses variation in the resistance of the photocell that detects light 34 emitted from the bulb 16 passing through an opening in the reflector 28 to contact the photocell, is used to detect an inoperative bulb. The resistivity of the photocell 32 changes, which is sensed by the associated circuit 33, to produce a control signal which is applied to the magnetron controller (not illustrated). The magnetron controller functions, when the photocell indicates that light is not being received, to disconnect the electrical power from the magnetron 12 to thereby turn it off.
A typical microwave-powered UV lamp is six or ten inches in length and incorporates one or more magnetrons 12, as illustrated in FIGS. 1 and 2, to provide microwave power to excite a bulb 16. When the lamp 10 is first turned on, there is a high voltage standing wave ratio (VSWR) seen by the magnetron 12 because the bulb 16 is cool and the impedance of the bulb at the microwave frequency is not well matched. As the bulb 16 warms up, the VSWR gradually decreases to a steady state value as illustrated in FIG. 8. High VSWR transients are a normal part of starting ignition of a plasma-filled bulb 16. During the transient period, it is possible for arcing to occur in the microwave cavity/waveguide 14 which can destroy the magnetron. The magnetron may also experience severe anode dissipation during the transient period.
If there is no protection provided by the photocell 32 and protection circuit 33, the VSWR will remain unacceptably high until the magnetron is destroyed. The power supply (not illustrated) of the magnetron 12 applies high voltage to the magnetron. The magnetron controller is responsive to a signal from the protection circuit 33. If no signal is received indicating ignition within a set period of time, the magnetron controller assumes the bulb has failed to ignite and cuts electrical power to protect the magnetron 12 given the fault condition indication of no ignition.
The use of a photocell 32 and protection circuit 33 is subject to being unable to distinguish between light 34 from the bulb 16 and stray ambient light which enters the housing 22 from other sources. The presence of ambient light may result in the photocell 32 and protection circuit 33 sensing the ambient light from another source as an indication that the bulb has ignited. If in fact the bulb 16 has not ignited, damage to the magnetron 12 will occur by its continuing to provide output power the bulb 16 which is not absorbed by the plasma therein. Furthermore, the photocell 32 and control circuit 33 may fail requiring repair resulting in costly downtime for any system relying upon the operation of the microwave powered lamp and furthermore, if repair is not made, the probability of the magnetron 12 being permanently damaged by power being applied thereto is high.